1. Field of the Invention
The present invention generally relates to hip-replacement surgery using a computer-assisted surgery (CAS) system and, more particularly, to a method for orienting an acetabular implant for a CAS system.
2. Background Art
In hip-replacement surgery, optimal orientation of the pelvic prosthesis implant (also known as “acetabular implant” and “acetabular cup”) in the acetabulum reduces the risks of limb length discrepancy and dislocation due to implant impingement.
In order to guide the surgeon in setting the acetabular implant in an optimal position, a CAS system provides position and orientation information during the operative steps. The information may be provided in the form of values including anteversion and inclination.
The anteversion and inclination values are related to a frame of reference of a patient. A frame of reference is defined preoperatively or intraoperatively, and this frame of reference is used as a reference for the anteversion and inclination values.
Various methods have been described to define frames of reference, some of which include forming digital planes on a pelvis from points digitized at predetermined landmarks. In the latter case, a certain level of reproducibility is attained from patient to patient as the predetermined landmarks are present on every patient. This level of reproducibility allows anteversion and inclination values to be compared from patient to patient, and surgeons may set the acetabular implant orientation of a patient within ranges of orientation he/she is familiar with.
The frames of reference currently used in CAS do not take into account the pelvic orientation with respect to the natural postures. For instance, a patient may have an abnormal pelvic orientation when standing straight, when lying. The pelvic orientation or pelvic tilt will have a direct effect on the actual anteversion and inclination values of the patient once the patient takes these postures.